1. Field of the invention:
This invention relates to a semiconductor laser device which attains laser oscillation in a stabilized fundamental transverse mode at an extremely low threshold current level.
2. Description of the prior art:
As an example of semiconductor laser devices which oscillate a laser beam at a low threshold current level, there is a buried heterostructure (BH) semiconductor laser device, in which a laser-oscillating active layer is buried with burying layers having a lower refractive index than that of the active layer. The BH semiconductor laser device oscillates a laser beam according to an index waveguiding operation and has a low threshold current of 20 mA or less.
However, the BH semiconductor laser device has the disadvantage of oscillating a laser beam in an unstabilized transverse mode because the burying layers cannot be grown outside the active layer in a striped shape under stable conditions. Moreover, when coupled to an optical system such as an optical fiber, the BH semiconductor laser device has a tendency to produce excess noise because of its high sensitivity with respect to feeble light reflected from an optical system coupled thereto. Thus, the BH semiconductor laser device cannot be used as a replaying light source for compact disk players, which is required to have excellent low-noise characteristics, nor as a replaying light source for video disk players, which is required to have still more excellent low-noise characteristics than those of a replaying light source for the compact disk players.
In contrast to such a BH semiconductor laser device, a V-channeled substrate inner stripe (VSIS) semiconductor laser device has been proposed, in which light absorbing areas are disposed on both sides of a striped index-guiding waveguide. (See, e.g., Appl. Phys. Lett. 40 p. 372 (1982).) FIG. 4 shows a conventional VSIS semiconductor laser device comprising an n-GaAs current blocking layer 42 formed on a p-GaAs substrate 41. On the center portion of the n-GaAs current blocking layer 42, a V-striped channel is formed in the GaAs substrate 41 through the n-GaAs current blocking layer 42, and a p-GaAlAs cladding layer 43 is disposed on the n-GaAs current blocking layer 42 including the V-striped channel. The upper face of the p-GaAlAs cladding layer 43 is flat, on which a p-GaAlAs active layer 44, an n-GaAIAs cladding layer 45, and an n-GaAs cap layer 46 are successively formed. On the upper face of the n-GaAs cap layer 46 and the back face of the p-GaAs substrate 41, an n-sided electrode 47 and a p-sided electrode 48 are disposed, respectively.
Such a VSIS semiconductor laser device has an advantage that crystal layers can be readily grown on the substrate by liquid phase epitaxy. Although the threshold current of this VSIS semiconductor laser device is about 40 mA, which is extremely higher than that of the BH semiconductor laser device, the VSIS semiconductor laser device is suitable for use as a light source for compact disk players because undesirable noise resulting from reflected light from an optical system coupled thereto can be suppressed.
When the VSIS semiconductor laser device is used as a light source for video disk players which is required to have still more excellent low-noise characteristics than those of a light source for compact disk players, the thicknesses of the cladding layer and the active layer are controlled to produce a self-pulsation phenomenon which occurs by the interaction between the carriers and the laser light in the active layer. The self-pulsation provides a shorter coherence length of the laser beam, so that the sensitivity with respect to reflected light from an optical system can be reduced, thereby attaining excellent low-noise characteristics. However, because the VSIS semiconductor laser device is usually produced by liquid phase epitaxy, the thicknesses of the cladding layer and the active layer cannot be readily controlled. In liquid phase epitaxy, an error rate of about .+-.10% is unavoidable in the thickness of crystal layers grown. Therefore, when the VSIS semiconductor laser device is produced by liquid phase epitaxy, a production yield of self-pulsation semiconductor laser devices is decreased.
In recent years, as a method by which crystal layers can be grown to have a substantially uniform thickness, a molecular beam epitaxy (MBE) method and a metal-organic vapor phase epitaxy (MOVPE) method have been developed. However, when such an MBE or MOVPE method is used, because a crystal layer is grown to have a substantially uniform thickness over the surface of an underlying layer, it is difficult to grow the crystal layer on a current blocking layer 42 having a V-shaped channel formed therein so that the crystal layer fills in the V-shaped channel and the upper face of the crystal layer becomes flat, as in the VSIS semiconductor laser device. For this reason, with the use of the MBE or MOVPE method, the cladding layer and the active layer cannot be grown with their thicknesses controlled, to produce a self-pulsation phenomenon in the VSIS semiconductor laser device.